Defect propagation resistant stretch films

ABSTRACT

A defect propagation resistant stretch film comprising a polymer of linear low density polyethylene and one or more copolymers selected from the group consisting of low density polyethylene, very low density polyethylene, and ultra low density polyethylene.

BACKGROUND AND OVERVIEW

The use of thermoplastic stretch wrap films for the overwrap packagingof goods, and in particular, the unitizing of palleted loads is acommercially significant application of polymer film, includinggenerically, polyethylene. Overwrapping a plurality of articles toprovide a unitized load can be achieved by a variety of techniques. Inone procedure, the load to be wrapped is positioned upon a platform, orturntable, which is made to rotate and in so doing, to take up stretchwrap film supplied from a continuous roll. Braking tension is applied tothe film roll so that the film is continuously subjected to astretching, or tensioning, force as it wraps around the rotating load inoverlapping layers. Generally, the stretch wrap film is supplied from avertically arranged roll positioned adjacent to the rotating palletload. Rotational speeds of from about 5 to about 50 revolutions perminute are common. At the completion of the overwrap operation, theturntable is completely stopped and the film is cut and attached to anunderlying layer of film employing tack sealing, adhesive tape, sprayadhesives, etc. Depending upon the width of the stretch wrap roll, theload being overwrapped can be shrouded in the film while the verticallyarranged film roll remains in a fixed position. Alternatively, the filmroll, for example, in the case of relatively narrow film widths andrelatively wide pallet loads, can be made to move in a verticaldirection as the load is being overwrapped whereby a spiral wrappingeffect is achieved on the packaged goods.

Another wrapping method finding acceptance in industry today is that ofhand wrapping. In this method, the film is again arranged on a roll,however, it is hand held by the operator who walks around the goods tobe wrapped, applying the film to the goods. The roll of film so used maybe installed on a hand-held wrapping tool for ease of use by theoperator.

Historically, higher performance stretch films have been prepared withm-LLDPE, most often with the m-LLDPE located in an interior layer. Suchfilms have shown markedly improved puncture and impact resistance aswell as improved film clarity relative to counterparts made with moretraditional Ziegler-Natta LLDPE's. Stretch films employing higheramounts (up to 100 wt %) of m-LLDPE either as a discrete layer orlayers, or as a blend component in a discrete layer or layers of amultilayer stretch film, propagate defects more easily leading to webbreakage. This defect propagation has precluded the development of filmstructures containing higher concentrations of m-LLDPE to maximizetoughness. The results of this work show that stretch films withsignificantly improved defect propagation resistance relative to thefollowing film types can be made: 1) A five-layer (A/B/C/B/A) stretchfilm formulation common in the stretch film industry whereA=C=Ziegler-Natta ethylene-hexene LLDPE and B=m-LLDPE; 2) A stretch filmcomprised of 100 wt % ethylene-hexene or 100 wt % ethylene-octeneZiegler-Natta copolymer.

TEST METHODS EMPLOYED

Film Testing Methods: MD and TD refer to the machine direction andtransverse direction, respectively, as they relate to cast filmproduction. Film Gauge (Exxon PLFL-238.001), Laboratory Puncture Force(Exxon PLFL-201.01), Elmendorf Tear (ASTM D1922-94); Cling (ExxonPLFL-201.02 based on ASTM D5458-95), Melt Index (ASTM D1238-94), Density(ASTM D1505-96, compression molding of samples by ASTM D1928-96), FDAHexane Extractables (21 CFR 177.1520(d)(3)(ii)). The Highlight UltimateStretch Test and the Highlight Puncture Test were each conducted inaccord with Highlight Industries, Inc. Film Development Test SystemOperations Manual (Copyright, 1996).

SUMMARY

In this work, a defect was introduced by way of a Highlight StretchTester Puncture Test (at progressively higher levels of stretch).Destruction of the web was designated by us as Failure Mode (FM) 3, thetype of film failure deemed most undesirable because web failurerequires more operator attention and effort in stretch wrap applicationsas those skilled in the art of stretch film know. FM2 referred to a filmpuncture, but no defect propagation. While still undesirable, the webwas not destroyed, merely damaged by a hole in this mode of failure. InFM1 the probe did not puncture the film and would be the most desirable.

This research disclosure shows how 5-layer film structures can beprepared with higher levels of m-LLDPE, particularly in the skin andcore film layers, that do not propagate a defect (FM3) as discussedabove, thereby maintaining the integrity of the web during use. Otherperformance benefits were noted from certain film structures prepared inthis work such as an optimal balance of stiffness and extensibility, aminimization in cling force reduction upon stretching, and aminimization in unwind force. Some film samples (such as samples 004 and007) were highly extensible, and required higher levels of force tostretch the film in the Highlight Ultimate Stretch Test. In someapplications, this combination of stiffness and extensibility ispreferred. Also noted in this work was that the reduction in cling forcewith film stretching generally experienced by those skilled in stretchfilm was minimized for some film samples. By way of example, cling forceat 200% stretch was higher for samples 004-005 than for samples 014-015.Most noteworthy was the higher cling force at 200% stretch of samples001-011 relative to sample 013, prepared from ethylene-octene LLDPE.Finally, while the relationship between unwanted increases in unwindforce and higher extractables concentrations are well known to thoseskilled in the art of stretch film, some of the film samples preparedherein exhibited dramatic increases in unwind force and unwind noiseunrelated to extractables. The root cause of the unwind force increasewas strain-induced crystallization. Steps to mitigate strain-inducedcrystallization and thus avoid unwanted increases in unwind force willbe addressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the general film structure.

FIG. 2 is the defect propagation resistance test results using highlightstretch tester puncture test at varying levels of stretch percent.

FIG. 3 is the comparison of experimental films and common formulations.

FIG. 4 is the lab puncture response plot.

FIG. 5 is the comparison of experimental films to common formulations.

FIG. 6 is the MD Elmendorf tear response plots.

FIG. 7 is the TD Elmendorf tear run plot.

FIG. 8 is the TD Elmendorf tear response plots.

FIG. 9 is the stress-strain curve for common film formulations.

FIG. 10 is the stress-strain curves for experimental films 1-6.

FIG. 11 is the stress-strain curves for experimental films 7-12.

FIG. 12 is the comparison of experimental films stiffness to commonformulations.

FIG. 13 is the highlight plateau stretch force response plot.

FIG. 14 is the highlight ultimate stretch test: response plot forelongation at break.

FIG. 15 is the cling at 0 and 200% stretch.

FIG. 16 is the response plot for highlight ultimate stretch test unwindforce at break.

POLYMER PROPERTIES AND FILM COMPOSITION

A summary of the polymer properties used in making the film samplesdescribed herein can be found in Table 1. The composition of each filmprepared in this work is given in Table 2.

TABLE 1 Properties of polyethylene polymers used and key to polymer IDin graphs Name in Grade Melt Index Density Catalyst Process Graphs Name(MI, dg/min) (g/cc) Comonomer Type Type m-LLDPE1 Exceed™ 3.4 0.9171-n-Hexene Metallocene Gas phase 357C32 (Exxpol™) LLDPE1 Escorene™ 3.00.917 1-n-Hexene Ziegler-Natta Gas phase LL-3003.32 LLDPE2 Escorene™ 2.00.917 1-n-Butene Ziegler-Natta Gas phase LL-1002.32 m-plastomer Exact™4.5 0.873 1-n-Butene Metallocene Gas phase 4049 (Exxpol™) LLDPE3 Dowlek™2.3 0.917 1-n-Octene Ziegler-Natta Solution 3347A phase LLDPE4 Escorene™2.35 0.917 1-n-Hexene Ziegler-Natta Gas phase LL-3023.32 HDPE Escorene™0.45 0.9580 — Mitsui Slurry Slurry HD-7845.30 LDPE Eswrene™ 3.0 0.921LD- Free radical High pressure LD-135.09 homopolymer

TABLE 2 Film Composition Film Sample 003,006,009¹ 001 002 004 005 007 ID→ (A/B/C/B/A) (A/B/C/B/A) (A/B/C/B/A) (A/B/C/B/A) (A/B/C/B/A)(A/B/C/B/A) (A/B/C/B/A) 10/20/40/20/10 10/15/50/15/10 10/25/30/25/1010/15/50/15/10 10/25/30/25/10 10/15/50/15/10 Layer Ratios (wt %) Layer A80 Layer A 80 Layer A 80 Layer A 80 Layer A 80 Layer A 80 wt % m-LLDPE1wt % m-LLDPE1 wt % m-LLDPE1 wt % m-LLDPE1 wt % m-LLDPE1 wt % m-LLDPE1Layer A 20 Layer A 20 LayerA 20 Layer A 20 Layer A 20 Layer A 20 wt %m-plastomer wt % m-plastomer wt % m-plastomer wt % m-plastomer wt %m-plastomer wt % m-plastomer Layer B 2.5 Layer B 100 Layer B 100 Layer B5 Layer B 5 Layer B 100 wt % HDPE wt % LLDPE1 wt % LLDPE1 wt % HDPE wt %HDPE wt % m-LLDPE1 Layer B 97.5 Layer C 100 Layer C 80 Layer B 95 LayerB 95 Layer C 80 wt% LLDPE2 wt % m-LLDPE1 wt % m-LLDPE1 wt % LLDPE1 wt %LLDPE1 wt % m-LLDPE1 Layer C 90 Layer C 20 Layer C 80 Layer C 100 LayerC 20 wt % m-LLDPE1 wt % LDPE wt % m-LLDPE1 wt % m-LLDPE1 wt % LDPE LayerC 10 Layer C 20 wt % LDPE wt % LDPE Film Sample 008 010 011 013 014 015ID → (A/B/C/B/A) (A/B/C/B/A) (A/B/C/B/A) (A/B/C/B/A) (A/B/C/B/A)(A/B/C/B/A) Layer Ratios (wt %) 10/25/30/25/10 10/15/50/15/1010/25/30/25/10 10/20/40/20/10 10/20/40/20/10 10/20/40/20/10 Layer A 80Layer A 80 Layer A 80 Layer A 100 Layer A 100 Layer A 100 wt % m-LLDPE1wt % m-LLDPE1 wt % m-LLDPE1 wt % LLDPE3 wt % LLDPE4 wt % LLDPE4 Layer A20 Layer A 20 Layer A 20 Layer B 100 Layer B 100 Layer B 100 wt %m-plastomer wt % m-plastomer wt % m-plastomer wt % LLDPE3 wt % LLDPE4 wt% m-LLDPE1 Layer B 100 Layer B 95 Layer B 95 Layer C 100 Layer B 100Layer C 100 wt % m-LLDPE1 wt % m-LLDPE1 wt % m-LLDPE1 wt % LLDPE3 wt %LLDPE4 wt % LLDPE4 Layer C 100 Layer B 5 Layer B 5 wt % m-LLDPE1 wt %HDPE wt % HDPE Layer C 100 Layer C 80 wt % m-LLDPE1 wt % m-LLDPE1 LayerC 20 wt % LDPE ¹Film Samples 003, 006 and 009 were replicatecenterpoints. In Figs. That follow, these points are collectivelyreferred to as Film Sample ID “0”.

Film Extrusion Conditions

All films were prepared on a Black Clawson cast film extrusion lineequipped with a 42″ wide Cloeren die and five-layer A/B/C/B/A feedblock. Typical LLDPE barrier screws were employed. The B/C layer ratiowas varied by changing relative extruder output while holding the totalthroughput constant at 550 lb/hr. The die gap was set to 20 mil. Dietemperatures were set to 550-555° F. The melt curtain length was 3.75inches long. The primary and secondary chill roll temperature inletwater temperatures were 70° F. and 72° F., respectively. The targetgauge of 0.80 mil was achieved with the range of film gauges detectedbetween 0.79-0.91 mil. Line speed was held constant at 699-701 ft/min.The film rolls were trimmed to 20″ in width prior to winding. No trimwas recycled. Melt temperatures: Layer A (544-547° F.), Layer B(551-562° F.), Layer C (556-575° F.). A winder tension of 8 lb. for thetrimmed 20-inch rolls was employed.

The films made in this work were cast, 0.8 mil thick, although gauges of0.5-1.5 mil could be employed without departing from the spirit of thiswork. The films were comprised of five layers of the structure typeA/B/C/B/A as shown in FIG. 1, a general film structure, where Layer A=20wt % (10 wt % per layer), but Layer A could vary 3-5 wt % to 30-40 wt %of the total film structure without departing from the spirit of thiswork. Layer A was comprised of 80 wt % m-LLDPE1 blended with 20 wt %m-plastomer, however, the concentration of m-plastomer could vary 3 wt%-40 wt % without departing from the spirit of this work. Layer B andLayer C were each blends of a major component and a minor component. InLayer B, the identity of the major was either LLDPE1, LLDPE2, orm-LLDPE1. The minor component in Layer B was always HDPE and varied inconcentration 0-5 wt %, however, concentrations of 0-30 wt % would alsobe acceptable. The major component of Layer C was always m-LLDPE1. Theminor component of Layer C was always LDPE and varied in concentration0-20 wt %, although concentrations of 0-100 wt % would also beacceptable without departing from the spirit of this work.

M-LLDPE is often referred to as a low polydispersity polymer by virtueof the narrow molecular weight distribution imparted with single sitecatalysis during polymer production. While a particular m-LLDPE wasemployed in this work, other types of m-LLDPE ranging in MI from 0.5-15dg/min, and density of 0.910-0.925 g/cc could be employed withoutmaterially changing the properties of the films. The low polydispersitym-LLDPE may be prepared with ethylene and at least one alpha olefinmonomer, e.g., a copolymer or terpolymer. The alpha olefin monomergenerally has from about 3 to about 12 carbon atoms, preferably fromabout 4 to about 10 carbon atoms, and more preferably from about 6 toabout 8 carbon atoms. The alpha olefin comonomer content is generallybelow about 30 weight percent, preferably below about 20 weight percent,and more preferably from about 1 to about 15 weight percent. Exemplarycomonomers include propylene, 1-butene, 1-pentene, 1-hexene,3-methyl-1-pentene, 4-methyl-1-pentene, 1-octene, 1-decene, and1-dodecene.

The low polydispersity m-LLDPE generally has the characteristicsassociated with an LLDPE material, however it has improved properties asexplained more fully below. The low polydispersity m-LLDPE definedherein has a density of from about 0.88 to about 0.94 g/cc, preferablyfrom about 0.88 to about 0.93 g/cc, and more preferably from about 0.88to about 0.925 g/cc.

The average molecular weight of the low polydispersity m-LLDPE cangenerally range from about 20,000 to about 500,000, preferably fromabout 50,000 to about 200,000. The molecular weight is determined bycommonly used techniques such as size exclusion chromatography or gelpermeation chromatography. The low polydispersity m-LLDPE should have amolecular weight distribution, or polydispersity, (M_(w)/M_(n), “MWD”)within the range of about 1 to about 4, preferably about 1.5 to about 4,more preferably about 2 to about 4, and even more preferably from about2 to about 3. The ratio of the third moment to the second moment,M_(z)/M_(w), is generally below about 2.3, preferably below about 2.0,and more typically in the range of from about 1.6 to about 1.95. Themelt flow ratio (MFR) of these polymers, defined as I₁₀/I₂ and asdetermined in accordance to ASTM D1238, is generally from about 12 toabout 22, preferably from about 14 to about 20, and more preferably fromabout 16 to about 18. The melt index (MI), defined as the I₂value,should be in the range of from about 0.5 to about 10 g/10 min.,preferably from about 1 to about 5 g/10 min. as determined by ASTMD1238. Useful low polydispersity m-LLDPEs are available from, amongothers, Dow Chemical Company and Exxon Chemical Company who areproducers of single site or constrained geometry catalyzedpolyethylenes. These polymers are commercially available as the AFFINITYand EXACT™ polyethylenes (see Plastics World, p.33-36, January 1995),and also as the ENHANCED POLYETHYLENE and EXCEED™ line of resins. Themanufacture of such polyethylenes, generally by way of employing ametallocene catalyst system, is set forth in, among others, U.S. Pat.Nos. 5,382,631, 5,380,810, 5,358,792, 5,206,075, 5,183,867, 5,124,418,5,084,534, 5,079,205, 5,032,652, 5,026,798, 5,017,655, 5,006,500,5,001,205, 4,937,301, 4,925,821, 4,871,523, 4,871,705, and 4,808,561,each of which is incorporated herein by reference in its entirety. Thesecatalyst systems and their use to prepare such copolymer materials arealso set forth in EP 0 600 425 A1 and PCT applications WO 94/25271 and94/26816. The low polydispersity polyethylene polymers thus producedgenerally have a crystalline content in excess of at least 10 weightpercent, generally in excess of at least 15 weight percent.

The above patents and publications generally report that these catalystscontain one or more cyclopentadienyl moieties in combination with atransition metal. The metallocene catalyst may be represented by thegeneral formula C_(c)MA_(a)B_(b) wherein C is a substituted orunsubstituted cyclopentadienyl ring; M is a Group 3-10 metal orLanthanide series element, generally a Group IVB, VB, or VIB metal; Aand B are independently halogen, hydrocarbyl group, or hydrocarboxylgroups having 1-20 carbon atoms; a=0-3, b=0-3, and c=1-3. The reactionscan take place in gas phase, high pressure, slurry, or solutionpolymerization schemes.

The low polydispersity m-LLDPE could be employed in any film layer orlayers in film structures having as few as three layers or as many asnine layers. Several specific types of LLDPE were employed in this work,however, others could also be used without materially altering filmproperties.

In this work, at least one of the film layers comprises a lowpolydispersity m-LLDPE which is blended with an LDPE resin. The LDPE inTable 1 refers to an ethylene homopolymer prepared from high pressure(greater than 1000 psi) free radical processes. Comonomers could also beemployed such as olefins in the C₃-C₂₀, vinyl esters in the C_(3-C) ₂₀range, and/or acrylic acid derivatives in the C₃-C₂₀ range. The resultof such polymerizations would be copolymers such as EVA, EMA, EEA, EnBAand so on. These LDPE copolymers would also be acceptable herein withoutmaterially changing the desired film properties. The LDPE resins have adensity of from about 0.9 to about 0.935 g/cc and preferably a densityof from about 0.915 to about 0.925 g/cc. The LDPE resins have a meltindex (I₂) of from about 0.5 to about 10, preferably from about 1 toabout 5, and most preferably from about 2 to about 4.0 g/10 min. TheLDPE resins comprise from about 0 to about 30 wt. % of a polymeric layeror layers. It is preferred to maintain the level of the lowpolydispersity m-LLDPE to at least 70 weight percent of a polymericlayer or layers. Additional, material may be incorporated with the blendof the low polydispersity polymer and the LDPE resin.

While Layer B was chosen to carry the LLDPE in this study, any otherfilm layer or layers in any film structure with as few as three or asmany as nine film layers could have been employed without materiallyaltering film properties as those skilled in stretch film know. As inother studies, the incorporation of LDPE into a film structure can leadto a desirable combination of film stiffness (high stretch force) andextensibility (elongation at break). Those persons skilled in stretchfilm know that there is an optimal concentration and type of LDPE thatcould be employed in stretch film. While a 3 MI, 0.921 g/cc density LDPEwas used in this work, other LDPE's as described in the paragraphs abovecould be used without materially altering film properties. Those personsskilled in stretch film also know that the film structure, number oflayers, thickness of the LDPE-containing layer, and choice of layers (A,B, C. . . etc.) to blend in LDPE are all important factors affecting thefinal properties of the stretch film.

Alternatively, the film layers could contain a blend of a lowpolydispersity m-LLDPE and a VLDPE resin such as the m-plastomerdescribed in Table 1. The m-plastomers are resins with a density rangingfrom about 0.86 to about 0.912 g/cc, more commonly from about 0.87 toabout 0.91 g/cc, and a melt index as I₂ of from about 0.5 to about 8g/10 min., and preferably from about 1 to about 5 g/10 min. Them-plastomer comprises from about 2 to about 30 wt. % of a polymericlayer with the preferred range of from about 5 to about 25 wt. % of theouter polymeric layer or layers. Additional materials may beincorporated with the blend of the low polydispersity m-LLDPE and them-plastomer resin.

While a specific type of m-plastomer was chosen for this study, othertypes of ultra low density polyethylene (ULDPE) or VLDPE (made from anynumber of different catalyst types) or polyisobutylene, or atacticpolypropylene could also have been used without materially changing thedesired film properties. These very low density materials could havebeen blended with any of the other components described herein, in anylayer of a stretch film containing as few as three or as many as ninefilm layers in proportions from 2 wt %-40 wt % without materiallychanging the film properties. In this particular example, the outerlayers of a five-layer film structure were chosen to contain them-plastomer. Those skilled in stretch film will appreciate andunderstand the benefits that use of the m-plastomer will bring tostretch film performance such as cling enhancement, impact and punctureresistance. LLDPE refers to ethylene-butene, ethylene-hexene, orethylene-octene copolymersprepared from Ziegler-Natta catalysts in gasphase, solution phase or slurry processes. LLDPE's with a MI ranging0.5-20 dg/min, density ranging 0.910-0.935 g/cc, comonomer type C₃-C₈,made in either a solution or gas phase process would be acceptablewithout departing from the spirit of this work. Suitable LLDPEs includethose having a density greater than about 0.900 g/cc, more preferably inthe range of from about 0.900 to about 0.940g/cc. The LLDPEs may alsohave wide ranging MIs, generally up to about 30 g/10 min., preferablybetween about 0.5 to about 10 g/10 min. Such LLDPEs and methods formaking the same are well known in the art and are readily availablecommercially under trade names such as Escorene™ LLDPE or Dowlex™ by wayof example.

The HDPE in Table 1 refers an to ethylene homo- or copolymers ofrelatively high molecular weight and relatively low comonomer contentprepared from, Ziegler-type catalysts or metallocene catalysts in gasphase, solution, or slurry processes. HDPE was employed as a blendcomponent in Layer B, but could equally have been employed in any otherlayer of a stretch film having as few as three or as many as ninelayers. The HDPE could be prepared from a variety of different processtypes. MI's 0.05-20 dg/min and densities 0.920-0.960 g/cc could beemployed without materially changing the properties of the films. MI'sin the range of 0.35-2 g/10 min. and densities of 0.94-0.96 g/cc aremost preferred. Other highly crystalline polymers could have also beenemployed such as polypropylene or polystyrene.

Results

Stretch films such as samples 003, 006 or 009 by way of example in Table3 proved both economically attractive and desirable in performance,particularly with respect to defect propagation resistance.Specifically, the structure of these A/B/C/B/A films was as follows:Layer A=80 wt % m-LLDPE1 blended on-line with 20 wt % m-plastomer; LayerB=97.5 wt % LLDPE2 blended on-line with 2.5 wt % HDPE; Layer C=80 wt %m-LLDPE1 blended on-line with 10 wt % LDPE. The layer ratios (wt %) ofthese A/B/C/B/A films were 10/20/40/20/10, respectively. Other ratios ordifferent polymers such as described in the preceding paragraph couldalso be used without departing from the spirit of this work and would beknown to those skilled in the art of stretch film. For even higherlevels of performance, especially more extensibility without defectpropagation, stretch films such as samples 001, 002, 004 or 005 provedattractive. In these formulations, LLDPE2 was employed as the majorcomponent of Layer B. The concentrations of minor components in Layers Band C and A/B/C/B/A layer ratios varied amongst the film samples.

Experimental Design

This study was conducted from a planned matrix of factors in a 2⁽⁴⁻¹⁾fractional pattern. The factors studied are given in Table 3 below.

TABLE 3 Experimental Factors Factor Low Value High Value Center Point IDof Layer B LLDPE1 m-LLDPE1 LLDPE2 Weight ratio of B/C Layers 0.6 1.6 1wt % HDPE in Layer B 0 5 2.5 wt % LDPE in Layer C 0 20 10

The films produced within this matrix (film samples 001-012) arereferred to as “experimental films”. For comparison, three additionalfilms were produced. Film samples 013 and 014 were comprised of 100 wt %LLDPE3 and LLDPE4, respectively. These “monolayer” films were producedusing the same A/B/C/B/A selector plug employed in the production of theexperimental films; however, all extruders (A, B, and C) extruded anidentical resin. Film sample 015 was produced for comparative purposesand is a representative example of a multi-layer stretch film of thetype well known to those skilled in the art of stretch film. These threefilms are referred to as “common industry formulations”.

Defect Propagation Resistance Testing

Defect propagation resistance testing was carried out employing aHighlight Ultimate Stretch Tester Puncture Test. Films were tested usingthe puncture test at varying levels of stretch. The tests were conductedin accord with Highlight Stretch Tester product literature and operatingmanuals. The stretch levels for the puncture test were set progressivelyhigher until the film repeatedly tore off before the stretch level couldbe “locked in” by the tester at the desired stretch level precludingcompletion of a puncture test at the desired stretch level.

The results of defect propagation resistance testing for all films aregiven in FIG. 2, defect propagation resistance test results usinghighlight stretch test at varying levels of stretch percentage. Thesingle most important factor in mitigating defect propagation was forLayer B to not employ m-LLDPE1 as the major component. When the majorcomponent of Layer B was m-LLDPE1, no FM2 was observed only FM1 and FM3.During testing, the transition from FM1 to FM3 was striking for filmsamples 007-008, and 010-011. Either the films did not puncture at all,or they punctured leading rapidly to web destruction. No other factorswere more important in controlling defect propagation than the identityof the Layer B major component. Film samples 013-015 also did not showsigns of FM3, and generally tore off before any puncture testing couldtake place when higher levels of stretching were attempted. It wouldalso appear that if a film could be sufficiently stretched in the testwithout tearing off first, it could ultimately be forced into FM3 afterpassing though a region of FM2 at lower stretch values. Such was thecase where the major component of Layer B was LLDPE1 but not where themajor component was m-LLDPE1. Those films readily transitioned betweenFM1 and FM3 without any sign of exhibiting FM2. The raw data for allfilms are tabulated below.

Tabular Data for FIG. 2.

STRETCH FAILURE FILM SAMPLE ID (%) MODE MAJOR COMPONENT 1 315 2 LLDPE1 2275 1 LLDPE1 3 270 2 LLDPE2 4 340 3 LLDPE1 4 330 2 LLDPE1 4 320 2 LLDPE15 300 2 LLDPE1 6 270 1 LLDPE2 6 265 1 LLDPE2 7 285 1 m-LLDPE1 7 300 3m-LLDPE1 11 300 3 m-LLDPE1 11 300 3 m-LLDPE1 11 315 3 m-LLDPE1 8 285 1m-LLDPE1 8 300 3 m-LLDPE1 8 300 3 m-LLDPE1 10 265 3 m-LLDPE1 10 265 3m-LLDPE1 11 300 1 m-LLDPE1 11 275 1 m-LLDPE1 12 300 2 LLDPE2 12 290 2LLDPE2 12 285 2 LLDPE2 12 280 1 LLDPE2 12 270 2 LLDPE2 13 195 1 LLDPE313 180 1 LLDPE3 14 205 1 LLDPE4 15 235 1 LLDPE4 15 230 1 LLDPE4 1 310 2LLDPE1 1 310 3 LLDPE1 1 300 2 LLDPE1 1 280 1 LLDPE1 1 270 1 LLDPE1 2 3402 LLDPE1 2 330 2 LLDPE1 2 320 2 LLDPE1 2 300 2 LLDPE1 2 300 2 LLDPE1 3270 2 LLDPE2 3 265 1 LLDPE2 4 350 3 LLDPE1 4 320 2 LLDPE1 4 300 2 LLDPE14 300 2 LLDPE1 5 300 2 LLDPE1 5 300 2 LLDPE1 5 285 1 LLDPE1 5 270 1LLDPE1 6 270 2 LLDPE2 6 270 2 LLDPE2

Laboratory Puncture Testing

The test method followed for laboratory Puncture Resistance Tester was amodified ASTM D5748-95, as further described later. The run plot showsin FIG. 3, a comparison of experimental films and common formulations,that most of the experimental films produced in this work requiredsignificantly more force to puncture relative to the common filmformulations. Film samples 003, 006 and 012, the centerpoints in thisstudy, are shown as Film Sample ID “0” in FIG. 3 in order to give anestimate of the reproducibility of both the film fabrication and testingusing true replication and to show that the effects of the factors inthe designed experiment were larger than variations due to sampleproduction or film testing. Film Sample ID's 13 and 14 contain 100percent LLDPE and Film Sample ID 15 contains some m-LLDPE.

Tabular Lab Puncture Data for FIG. 3.

FILM SAMPLE ID PUNCTURE FORCE (lb/mil) 1 11.99 2 12.36 3 12.68 4 12.77 512.14 6 12.64 7 12.78 8 12.44 10 14.31 11 13.49 12 12.5 13 10.39 1410.49 15 11.49

The experimental lab puncture data above were analyzed with respect tothe experimental design factors given in Table 3. The response plot inFIG. 4, a lab puncture response plot, below shows that the combinationof 5 wt % HDPE plus 95 wt % m-LLDPE1 in Layer B gave a significantincrease in puncture force. These same films, however, also failedcatastrophically (FM3) as shown in FIG. 2. FIG. 4 suggests that thecombination of 5 wt % HDPE blended on-line with 95 wt % LLDPE2 in LayerB, with A/B/C/B/A layer ratios of 10/20/40/20/10 would be a reasonablygood alternative. Lab puncture force would be maximized according toFIG. 4 and defect propagation would be minimized according to FIG. 2.Additionally, this approach would be economically attractive becausebutene LLDPE's such as LLDPE2 are generally lower in cost. Other highlycrystalline polymers besides HDPE would also likely behave in a similarfashion, for example, highly crystalline polypropylene.

Elmendorf Tear Test Results

Elmendorf Tear testing was conducted by the method of ASTM D 1922-94.The run plot in FIG. 5, a comparison of experimental films to commonformulations shows that certain film formulations gave MD Tearproperties at least as good as common formulations in the industry. Asin FIG. 3, Film Sample ID are provided to give an estimate of thereproducibility of both the film fabrication and testing using truereplication. Also, as in FIG. 3, Film Sample ID's 13 and 14 are 100percent LLDPE and Film Sample ID contains some m-LLDPE.

Tabular data for MD Elmendorf Tear Data in FIG. 6 are given below.

FILM SAMPLE ID MD ELMENDORF TEAR (g/mil) 1 222 2 118 3 94 4 61 5 191 6127 7 77 8 195 10 245 11 105 12 130 13 213 14 202 15 178

The response plot in FIG. 6 shows that lower MD Tear values could beaccounted for by higher concentrations of LDPE in Layer C, the centerlayer. The highest concentration of LDPE in any experimental films was10 wt %.

In FIG. 6, the following ratios were used:

A/B/C/B/A LAYER RATIOS (%) B/C LAYER RATIOS 10/15/50/15/10 0.610/20/40/20/10 1.0 10/25/30/25/10 1.6

FIG. 7 shows the run plot for TD Tear. There are two noteworthy pointsregarding this run plot. First, film samples 007-008 and 010-011 had TDTear values comparable to film sample 015, yet they contained 86-96 wt %m-LLDPE1 while sample 015 contained only 40 wt % m-LLDPE. This suggeststhat TD Tear strength reduction and m-LLDPE concentration are notnecessarily correlated. Secondly, the centerpoint film samples (003, 006and 012) labeled film sample “0” in FIG. 7 were resistant to thepropagation of defects, whereas film samples 007-008 and 010-011 werenot. FIG. 2 showed film samples 003, 006 and 012 to be defectpropagation resistant and film samples 007-008 and 010-011 not to bedefect propagation resistant. These observations thus show that there isno correlation between TD Tear values and defect propagation resistancebecause TD Tear performance was comparable, but defect propagationresistance was not. Film Sample ID's 13 and 14 contain 100 percent LLDPEwhile Film Sample ID 15 contains a common industry polyethyleneformulation.

Tabular data for MD Elmendorf Tear data in FIG. 7 are given below.

MD ELMENDORF TD ELMENDORF FILM SAMPLE ID TEAR (g/mil) TEAR (g/mil) 1 222655 2 118 798 3 94 652 4 61 701 5 191 779 6 127 600 7 77 597 8 195 57110 245 651 11 105 640 12 130 616 13 213 827 14 202 812 15 178 624

Analysis of the experimental data showed that the concentration of LDPEin Layer C was not a significant factor affecting TD Tear, unlike the MDTear response. Rather, FIG. 8 shows that the identity of the majorcomponent in Layer B was most important, as well as the B/C layer ratio.TD Tear values were highest when the major component of Layer B wasLLDPE1. Tear values decreased when the Layer B major component wasLLDPE2 or m-LLDPE1.

In FIG. 8, the following ratios were used:

A/B/C/B/A LAYER RATIOS (%) B/C LAYER RATIOS 10/15/50/15/10 0.610/20/40/20/10 1.0 10/25/30/25/10 1.6

Unique Combinations of Stiffness and Extensibility Achieved

Films with high proportions of m-LLPDE are generally perceived as softerand stretchier (higher Highlight Ultimate Stretch Test values) by thoseskilled in stretch film. It would appear that this perception of m-LLDPEhas limited its use in stretch film applications requiring higherstiffness. The end user perceives a stretchy film as one with poorload-holding capability in stretch wrapping applications. Whilestretchiness can be highly desirable, it is generally achieved at theexpense of stiffness and vice versa. This disclosure demonstrates howmultilayer stretch films (for example, 5-layer cast) can be preparedgiving films that are both stiff and highly extensible.

FIGS. 9-11 show stretch force versus stretch percent curves(stress-strain curves) taken from Highlight Ultimate Stretch Test data.Load cells between rollers that stretched the film as it unwound at 180ft/min measured the amount of force required to stretch the 20-inch filmrolls at progressively higher levels of stretch. Following the initialexponential rise in stretch force at lower stretch percent values(50-60%) on the stress-strain curve, the stress-strain curves reached aplateau stretch force. The stretch force remained relatively constantover a wide range of stretch percent values, hence the term of “plateaustretch force”. An example of the interpolation to determine plateaustretch force can be seen where a straight line was drawn through theplateau region of the stress-strain curve for film sample 004 in FIG.10. At higher stretch percent levels, which varied from film to filmbased on the factors in the study, the film strain-hardened reachingprogressively higher levels of stretch force and then finally broke.

FIG. 12 shows that many of the experimental films produced herein wereconsiderably stiffer, or higher in plateau stretch force, than filmsamples 013-015. The response plot in FIG. 13 shows that the mostimportant factor in increasing stiffness was the higher concentration ofLDPE in Layer C. Recall that the highest concentration of LDPE employedin any experimental film structure was 10 wt %. Additionally, the LDPEincreased the length of the plateau region resulting in higher filmextensibility (higher elongation at break) often referred to as“Highlight Ultimate Stretch” values as shown in FIG. 14. To the end userof stretch film, this combination of stiffness and extensibility shouldbe valuable because the film could be stretched substantially furtherwithout breakage when compared to common film formulations (013-015). Athigher stretch values, higher stiffness should translate into superiorload-holding capacity. As in previous figures, Film Sample ID Ø showstrue replicates to graphically demonstrate experimental error. Also, asin previous figures, Film Sample ID's 13 and 14 contain 100 percentLLDPE while Film Sample ID 15 contains some m-LLDPE.

In FIG. 14, the following ratios were used:

A/B/C/B/A LAYER RATIOS (%) B/C LAYER RATIOS 10/15/50/15/10 0.610/20/40/20/10 1.0 10/25/30/25/10 1.6

Plateau Stretch Force Tabular Data for FIG. 12.

FILM SAMPLE ID PLATEAU ULTIMATE STRETCH FORCE 2 74 3 71 4 78 5 63 6 71 781 8 64 10 61 11 77 12 69 13 60 14 61 15 63

Tabular Highlight Ultimate Stretch Test Elongation (%) at Break for FIG.13.

FILM SAMPLE ID Highlight Ultimate Stretch (Elongation at Break, %) 1 3042 354 3 321 4 384 5 308 6 312 7 381 8 305 10 279 11 358 12 311 13 287 14283 15 294

Reduction in Cling Force Upon Stretching Minimized

Historically, LLDPE produced in gas phase Ziegler-Natta polymerizationreactions, has enjoyed wide use in stretch film outer cling layers. Thishas been attributed to a particularly desirable level of “extractables”.One method of determining “extractables” levels is by the FDA hexaneextractables test. Blown monolayer film samples of nominal 3-milthickness were prepared on a small blown film line. The resulting filmsamples were submitted to a certified lab for the FDA hexaneextractables test along with a control film sample. Basically, the“extractables” value was determined by measuring the loss in weight of afilm sample after stirring in hexane at 50° C. for 2 hours. The hexaneextractables value was reported as an average of two determinations andwithin the context of a control film sample results. Typical“extractables” values for gas phase LLDPE's such as LLDPE3 would be inthe range of 2.7-4 wt %. It is also well known that commerciallyavailable ethylene-alpha-olefin copolymers prepared in solution phaseZiegler-Natta processes do not perform as well as their gas-phasecounterparts with respect to cling performance. This reduced performancehas most often been attributed to lower levels of “extractables”relative to gas phase Ziegler-Natta LLDPE counterparts.

The method employed for cling testing the film samples in this work bothat 0% and 200% stretch level was a modified ASTM D5458-95 as describedlater. It is well known that a depression in cling generally accompaniesfilm stretching. FIG. 15 shows that this reduction was minimized in thesamples made from a combination of 80 wt % m-LLDPE1 blended on-line with20 wt % m-plastomer in Layer A. In FIG. 15, Film Sample ID's 4 and 5 arefilms having the “A” or outer layer made from 80 weight percentm-LLDPE3. Film Sample ID 14 is a film having layer “A” of 100 percentLLDPE4. Film Sample ID 15 is a film having layer “A” of 100 percentLLDPE4.

Avoid Higher Unwind Force in Stretch Film

This disclosure shows how unwanted increases in stretch film unwindforce can be avoided and one reason for its occurrence. Unwind Force forthe 20-inch film rolls prepared in this work was defined as the amountof force required to unwind a roll of stretch film at the breaking pointduring the Highlight Ultimate Stretch Test. A pair of load cells on thetester measured the amount of force in pounds required to unwind thefilm roll. The average unwind force at break from five ultimate stretchtests was employed in determining unwind force in this work. Theprocedures used were consistent with the Highlight Ultimate StretchTester operating manual. This was viewed as an acceptable measurement ofUnwind Force because the value of Unwind Force at any given stretchvalue during the Highlight Ultimate Stretch test varied little from thevalue reported by the Highlight Tester when the stretch film broke.

Further crystallization of polymer after winding of a film roll can leadto substantial increases in unwind force. This is an important point asmost skilled persons in stretch film would generally attribute thisincrease in unwind force to an accidental change in film composition,particularly the concentration of “extractables” in the outer clinglayers of the film. The concentration of “extractables” in the filmsmade in this work was derived from butene comonomer content measured by¹H NMR spectra acquired on the individual film samples. Thus, in thisdata set, we are assured of minimal variations in composition orextractables that might otherwise account for the unusually largedifferences in unwind force between film samples.

FIG. 15 shows that the interaction between the concentration of HDPE inLayer B and the B/C Layer ratio led to the unwanted increase in unwindforce. Specifically, when the concentration of HDPE in Layer B washighest, and Layer B was at its thinnest, unwind force increasedsubstantially because of the interaction between these two factors. Thedata suggest that this interaction between factors caused strain-inducedcrystallization after winding which resulted in a “contraction” of thefilm in the MD direction, the primary axis of orientation and stress inthe films. As a result of this contraction in primarily MD, pressuredirected inwards toward the core of the film roll increased whichsqueezed adjacent film sheets together more tightly resulting in theobserved increase in unwind force. As can be seen from FIG. 16, adecrease in the concentration of HDPE Layer in B or an increase in theB/C Layer ratio reduced the unwind force.

For FIG. 16, the following layer ratios were used:

A/B/C/B/A LAYER RATIOS (%) B/C LAYER RATIOS 10/15/50/15/10 0.610/20/40/20/10 1.0 10/25/30/25/10 1.6

There are certainly other ways of causing a delay in completecrystallization of the film, such as insufficient removal of heat duringextrusion. There are also many other causes of increased unwind force,such as higher “extractables”. The results of this particular work;however, do highlight the importance of ensuring that the majority ofcrystallization occurs before film winding, and also the potentiallydeleterious effects of increased stress even in a specific film layer ofa multi-layer film structure.

Tabular data for Unwind Force (FIG. 16)

Ultimate Stretch Test Unwind Force at Break FILM SAMPLE ID (lb) 1 9.7 29.6 3 15.4 4 23.2 5 13.3 6 11.9 7 10.5 8 7.7 10 31 11 9.9 12 15.7 13 7.514 9.5 15 10.2

For FIG. 16, the following layer ratios were used:

A/B/C/B/A LAYER RATIOS (%) B/C LAYER RATIOS 10/15/50/15/10 0.610/20/40/20/10 1.0 10/25/30/25/10 1.6

Film Testing Methodology

MD and TD refer to the machine direction and transverse direction,respectively, as they relate to cast film production. All filmproperties reported in this work were normalized to the target thicknessof the film (0.8 mil, 1 mil=1/1000 in.). As with any process a certainamount of variation is to be expected, hence the normalization of filmproperties to film gauge. Typical gauge variation observed in this workwas between 0.74-0.86 mil. The film testing procedures employed in thiswork were similar to those called for in ASTM procedures. Because theASTM procedures were not exactly followed, however, the test resultscannot be regarded as meeting ASTM standards. Below, the ASTM proceduresfor each film test employed in this work are referenced along with thedifferences between the ASTM procedure and the method followed in thiswork. Defect Propagation Resistance testing on the Highlight UltimateStretch Tester is discussed separately in the body of the text. TheHighlight Ultimate Stretch Test and the Highlight Puncture Test wereeach conducted in accord with Highlight Industries, Inc. FilmDevelopment Test System Operations Manual (Copyright, 1996). Thefollowing testing methods were conducted in accord with ASTM proceduresand the results can be regarded as having met the ASTM requirements:Melt Index (ASTM D1238-94), Density (ASTM D1505-96), compression moldingof samples by ASTM D1928-96. FDA Hexane Extractables, mentioned brieflyin this work, was conducted by the method of 21 CFR 177.1520(d)(3)(ii).

Gauge Measurement

In this work, film gauge was measured with a Gauge Mic (Micrometer Mfr.Heidenhain) in a manner similar to ASTM D-374 Method C, but with thefollowing exception: The micrometer was calibrated to 0.01 mil annuallyby the vendor. The ASTM procedure calls for monthly calibration andcontrol charting using a recognized standard +/−10% of the smallestmicrometer measurement (0.001 mil) in this case.

Laboratory Puncture Resistance Test

The Puncture testing in this work was conducted on a United TestingMachine SFM-1. The testing procedure followed was similar to ASTMD-5748-95 with the following exceptions: 1) A 0.75-inch diameterelongated stainless steel probe with matte finish was employed. The ASTMtest method calls a 0.75-inch diameter pear-shaped Teflon probe. 2) Inour testing, two HDPE slip sheets each approximately 0.25 mils thicklying loosely on the surface of the test specimen were employed. Thiswas done in order to correct for any possible differences in punctureresistance in the testing of stretch wrap films related to differencesin the film area contacting the probe. These differences can arise ifdifferences in cling force between film samples exist. The ASTM testmethod does not call for the use of slip-sheets. 3) Gauge measurement.In our testing, we used the Gauge Mic Procedure mentioned above for theaverage thickness. In the ASTM method, the average of three readings inthe test area for each of the five specimens tested is employed tocalculate puncture resistance per mil of film. In our testing, we reportAverage Peak Load (lbs.) normalized to the film gauge, which isinterpreted as the maximum force achieved. Additionally, our methodreports Average Break Energy (inch lbs.) normalized to the film gauge.This value indicates the energy required to break the film. In the ASTMmethod, the Peak Force at Break (lbs) is also reported as well as theProbe Penetration Distance (in). We do not report probe penetration dataor Peak Force at Break.

Propagation Tear Resistance of Plastic Film & Thin Sheeting by PendulumMethod (Elmendorf Tear Test)

Films in this work were tested in a manner similar to that of ASTMD-1922-94a on a Thwing-Albert Elmendorf Digi-Tear with the followingexceptions: The micrometer foot pressure employed in film gaugemeasurement associated with the test was between 4.6 and 6.7 psi. Thispressure was lower than that called for by ASTM D-1922-94a which callsfor the pressure exerted by the gage or micrometer foot to be between160 and 185 kPa (23 and 27 psi). We normalized tear test results in thiswork to the film gauge, hence, the comments in the preceding paragraphson film gauge testing with a micrometer are also applicable here.

Peel Cling of Stretch Wrap Films (Cling Testing)

The cling testing was similar to ASTM method D-5458-95 with thefollowing exceptions: 1) Line grips with one flat rubber side and onecurved steel side were used to hold the string and the clip which pullsthe one-inch strip from the incline plane. The ASTM procedure (ASTMD-5458-95) calls for flat rubber sides on both grips. 2) Due to the linegrip apparatus weight we used a 20 lb load cell. The ASTM procedurecalls for a 500 gram load cell. A crosshead speed of 3.94 in/min (10.00cm/min) was employed. The ASTM method calls for a crosshead speed of 5in/min. 4) For 100% & 200% stretch cling values, we stretched both the1-in. test strip (top) and the specimen attached to the incline plane(bottom). The ASTM procedure calls only for stretching of the specimenon the incline plane (bottom) to the target stretch value. The teststrip (top) is not stretched in the ASTM method.

I claim:
 1. A film comprising a polymer of linear low densitypolyethylene (LLDPE) and one or more copolymers selected from the groupconsisting of low density polyethylene (LDPE), very low densitypolyethylene (VLDPE), and ultra low density polyethylene (ULDPE).
 2. Afilm comprising a polymer of linear low density polyethylene (LLDPE) andone or more copolymers selected from the group consisting of low densitypolyethylene (LDPE), very low density polyethylene (VLDPE), and ultralow density polyethylene (ULDPE), wherein the LLDPE comprises ethyleneand at least one alpha olefin monomer selected from the group consistingof propylene, 1-butene, 1-pentene, 1-hexene, 3-methyl-1-pentene,4-methyl-1-pentene, 1-octene, 1-decene, and 1-dodecene, the alpha olefinmonomer having a content generally below about 1 to about 15 wt %; theLLDPE having: a) a melt index of from about 1 to about 5 dg/min; b) adensity of from about 0.88 to about 0.94 g/cc; c) a molecular weight offrom about 50,000 to about 200,000; d) a molecular weight distributionof from about 2 to about 4; e) a ratio of the third moment to the secondmoment below from about 1.6 to about 1.95; and f) a melt flow ratio fromabout 16 to about 18.